The invention relates to new and useful improvements in aviation navigation systems and methods, and more particularly to aviation distance and bearing detection apparatus, and most particularly to tactical air navigation systems (TACAN).
In aircraft operations it is often desirable for two aircraft to rendezvous for joint operations. A good example is air-to-air refueling in which are aircraft requiring fuel rendezvous with a tanker aircraft. Various navigational methods, including TACAN have been used to direct the aircraft to a rendezvous point.
TACAN is a system used primarily by military services for transmitting and receiving range and bearing information by radio between a base station and a remote station installed in an aircraft. TACAN can operate in either a ground-to-air or an air-to-air mode. In the ground-to-air mode, the TACAN base station is located at a fixed location on the earth's surface where it is often co-located with a civilian VHF Omnirange (VOR) station to form what is called a VORTAC station. This permits civilian aircraft to use the TACAN range signal in distance measuring equipment (DME). In the air-to-air mode, to which the present invention is most applicable, the TACAN base station is placed in an aircraft.
To determine its range from a TACAN base station, an aircraft TACAN station interrogates the base station by sending a pair of interrogation radio frequency pulses of predetermined shape, duration and spacing on the TACAN frequency. The base station receives the interrogation pulses and after a predetermined delay transmits either a pair of pulses (ground-to-air mode) or a single response pulse (air-to-air mode). The remote aircraft TACAN station receives the response pulses and calculates the range from the timing difference between the transmitted and received pulses. In continuous operation the interrogation-reply process produces a low data rate of about 20-32 pulses per second. The range portion of TACAN is described in Bose, Keith H., Aviation Electronics, Howard H. Sams & Co. (4th Ed.-1981), p. 183, which is incorporated herein by reference.
To determine bearing from a remote aircraft station to a base station, the base station transmits a train of constant amplitude radio frequency pulses at a rate of 2700 pulses per second in the ground-to-air mode and 1300 pulses per second in the air-to-air mode. The base station combines the range reply pulses used in the range application with the bearing pulses so that a single integrated series of pulses is produced. The base station uses an antenna which has a cardioid pattern and rotates, either mechanically or electronically, the antenna pattern in an azimuthal direction at a rate of 15 Hz. From the perspective of a remote station, this produces an amplitude modulated pulsed carrier with a sinusoidally varying scan envelope with a frequency of 15 Hz. on the bearing pulse train. At the instant when the antenna pattern passes through cardinal East direction, the base station transmits a reference burst of 12 pulses having a unique, easily recognizable spacing. In the ground-to-air mode a 135 Hz. pulse overlay is also added atop the 15 Hz. envelope to provide increased bearing accuracy, but it is not critical for purposes of the present invention.
The remote aircraft TACAN station receives the bearing pulse train using an omnidirectional antenna. To a remote aircraft which is due East of the base station, the reference burst coincides with a maximum on the scan envelope. For remote aircraft at other bearings, the reference burst appears at other phase positions on the scan envelope with respect to the maximum. The bearing is thus related to the phase difference between the maximum on the scan envelope and the phase location of the reference burst. The remote aircraft TACAN station thus measures the fundamental bearing and combines this with the aircraft heading to calculate the relative bearing to the base station. Course information is added to the bearing information, and the TACAN system in the remote aircraft calculates course deviation and to-and-from information. The relative bearing, course deviation, and to-from data are normally applied to a horizontal situation indicator (HSI) in the aircraft, which displays the information for use by the flight crew.
In some air-to-air applications, it is desirable to operate TACAN in what is called the inverse bearing mode in which the omnidirectional antenna is installed on the airborne base station and the cardioid antenna with rotating pattern is installed on the remote aircraft station. In this mode the reference burst is not transmitted but is replaced by a reference pulse which is generated in the remote aircraft station by the antenna when the antenna pattern points to a convenient point on the aircraft, such as the aircraft nose. Otherwise, the inverse bearing mode operates in the same way as the normal mode, except that the bearing is determined relative to the aircraft rather than the cardinal points of the compass. This configuration can be used either in the air-to-air or ground-to-air mode, but it is most useful in the former. This configuration is particularly useful in an application such as the aircraft refueling scenario described above.
Current air-to-air rendezvous applications use the standard (not inverse) TACAN arrangement with a base station located in a tanker, for example. A major limitation drawback of this standard "flying beacon" configuration is that electrical power for operating the TACAN transmitter must be generated onboard the aircraft and is thus limited. The two pulses necessary for the range function do not require much power, but the bearing function requires a large amount of power to transmit 1300 pulses per second. Such large power levels necessitate a large heat removal capacity in the aircraft. Lower pulse rates could be used but typical current remote TACAN receivers require a minimum of about 800 pulses per second to extract usable data.
Another limitation of present air-to-air rendezvous operation using TACAN is that it is a one-way system for bearing information. Bearing transmit capability could be provided on all aircraft, but this would be very expensive, would suffer the aforementioned power limitations, and would compromise weight and space limitations. Rendezvous between aircraft not equipped with bearing transmit function is cumbersome since only one aircraft can actively seek the other. If aircraft are to actively move toward each other, radar must be used in conjunction with voice or other data communications.
Inverse TACAN has been used in air-to-air rendezvous applications but it suffers the same power limitations as the standard TACAN arrangement. However, if the power limitations could be overcome, it offers the prospect of using the approximately 15,000 active remote TACAN stations thus providing greater coverage and utility than the approximately 200 standard "flying beacon" installations.
It is therefore an object of the present invention to provide bearing information using only low data rate ranging information from a TACAN, or the like.
It is another object of the present invention to provide bilateral TACAN bearing operations.
It is a further object of the present invention to permit aircraft rendezvous more efficiently using a TACAN navigation system.